The issue with "Stupid" is that it's really heavily a value judgment, or at least can be. Stupid is bad. However, a lack of intelligence, knowledge, quick thinking, etc can be symptoms of disability, and even if not, are sometimes (usually, or often, depending on your perspective) things that the person has no control over. Plus, if you do something like call, say, an evil politician "Stupid," you're really missing the point, so in that case the label is a horrible mis-evaluation or mislabel of the problem, and then you're associating stupid people with Dick Cheney, who is really smart and really knows what he's doing, but is just...Darth Vader.

Yeah, that's another problem with the "I'm X and I think". It seeks to make it all about experience, when the opinions can be completely independent of experience. (And that's where a lot of the, "I don't care if nobody associates idiot with the intellectually disabled, you still shouldn't use it" arguments come from--sometimes just the history is enough.)